Rotary helically splined actuators have been employed in the past to achieve the advantages of high-torque output from a simple linear piston-and-cylinder drive arrangement. The actuators typically employed a cylindrical body with an elongated rotary output shaft extending from end to end coaxially within the body, with an end portion of the shaft providing the drive output. Disposed between the body and the shaft is a piston sleeve splined to cooperate with corresponding splines on the body interior and the output shaft exterior. The piston is reciprocally mounted within the body and has a head for the application of fluid pressure to one or the other opposing sides thereof to produce axial movement of the piston. The sleeve is elongated and coaxially receives the shaft therein.
As the piston linearly reciprocates in an axial direction within the body, the outer splines of the sleeve engage the splines of the body to cause rotation of the sleeve. The resulting linear and rotational movement of the sleeve is transmitted through the inner splines of the sleeve to the splines of the shaft to cause the shaft to rotate. Bearings are typically supplied to rotatably support one or both ends of the shaft relative to the body.
With such an arrangement, as the piston reciprocally moves from one axial direction to the other to produce relative rotational movement between the body and the shaft in response to application of fluid pressure to the piston head, backlash results from the slack existing between the intermeshing splines of the piston sleeve and the body and the intermeshing splines of the piston sleeve and the shaft. While accurate machining of the splines will reduce the backlash problem, this procedure substantially increases the manufacturing cost. Even with accurate machining, conventional machining techniques are virtually incapable of totally eliminating the slack which produces the backlash problem. Furthermore, to the extent more accurate tolerances produce actuator parts which fit tightly together and reduce slack, assembly of the actuator becomes difficult. While accurate machining reduces slack initially, should the splined parts wear during usage or otherwise lose their original tolerances, no means exist for elimination of the slack that develops without disassembly of the actuator and possible remachining or replacement of the splined parts.
It will therefore be appreciated that there has been a significant need for a fluid-powered rotary actuator which does not require exceptionally accurate machining of the torque-transmitting parts to eliminate slack that produces backlash. The actuator should be easy to assemble and provide means for substantially complete elimination of the slack causing the backlash problem after the actuator is assembled. Elimination of the slack should be accomplished in a simple manner without requiring disassembly of the torque-transmitting parts from the body. Furthermore, the means for elimination of the slack should, with one adjustment, simultaneously remove the slack existing between all of the torque-transmitting parts within the body which translate linear movement of the piston into rotational movement of the output member. Means should also be provided for automatic elimination of backlash during both assembly of the actuator and its subsequent use without requiring any adjustment by the user, particularly for actuators of small size which have space limitations. The present invention fulfills this need and further provides other related advantages.